Three-phase separator for a fluidized bed apparatus

ABSTRACT

The invention relates to a three phase separator which comprises a compartment for the substantial separation of the gas-phase, this compartment being equipped with at least one partition which is applied for an internal gaslift circulation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 07/581,135, filedSep. 10, 1990, now U.S. Pat. No. 5,230,794, which is a continuation ofapplication Ser. No. 07/254,053, filed Oct. 6, 1988, now abandoned.

The invention relates to anaerobic waste water purification using animproved fluidized-bed process and apparatus. Since the early seventiesanaerobic treatment of industrial waste-water has gained considerably inimportance resulting in the development of improved reactors with highbiomass concentration. Compared to other high rate anaerobic reactors(filter reactors, UASB reactors as major available representatives), thefluidized bed system has the main potential advantages of higherpurification capacity, no clogging in the reactor (as in filters), noproblem of sludge retention (as in UASB system if granular sludge is notobtained) and small volume and area requirements. The relatively highupward liquid velocity prevents clogging and moreover guarantees a goodcontact between influent and the biomass attached to the carrier. Thevertical construction and the relatively small size of the fluidized bedreactors makes a totally closed construction possible preventingemission of malodeurs. Optionally the use of (expensive) corrosionresistant materials is possible.

However, also disadvantages of anaerobic fluidized bed reactors areknown which are related to biolayer growth and subsequent controlthereof. At the bottom of the reactors often only bare carrier particlesare present, which implies that part of the reactor has no purificationactivity. Operational problems e.g. concerning the relatively longstart-up times are caused by starting up with bare carrier particles,which may need a period of 2-4 months to become overgrown.

For example, in the methanogenic phase, under full scale conditionsafter inoculation with suitable biomass the growing phase will takebetween 4 and 12 weeks. In this period of time the stability of thesystem is very sensible to, for example, peak loads. If the specificload is expressed as kg COD/kg VSS/day, then the load on the still smallamounts of biomass in the reactor will be too large, this may result inconsiderable losses of biomass. A short-duration incident may result inan almost complete new start-up of the system. Moreover, in connectionwith a preferred short residence time (see for example European PatentApplication EP-A-28846) during the start-up a pH control may benecessary. Another source of inconvenience is that during standstill ofthe reactor the liquid distributor may clog due to the settled carriermaterial at the bottom of the reactor.

Another type of anaerobic process, the UASB-systems (Upflow AnaerobicSludge Bed) is described in U.S. Pat. No. 4,253,956. Such a process iswell-known and often used in practice. On full scale a short andreproducible start up is possible, when sufficient granular sludge isadded before the starting up period under exactly predescribedconditions. The granular sludge may be obtained from other UASB-systems.This type of granular sludge consists of granules of active material,which are formed by nature in the UASB-reactor. These granules remainactive for years, may be removed from the reactor and may be stored,which makes these granules very suited for use as inoculation materialfor new plants, or for the re-starting of existing plants afterincidents. The UASB-system is also very suited for seasonal plants (forexample beet-sugar mills), in which the process is interrupted formonths while a fast re-starting up is essential.

However the UASB-reactors have some disadvantages as well, for example,the liquid velocity (1-2 m/h) is insufficient to prevent the (partial)sedimentation of inert sediments in the reactor. To prevent this inseveral cases the waste water has to be treated first in a primarysettler to separate this inert sediment. The UASR-reactors are rathersensible to air-inlet in the upper part, due to their construction,which may result in corrosion. Sometimes the emission of bad-smellinggases may take place due to their escape from the upper part. Theoverpressure on the upper part must be kept low which makes thetransport of the corrosive biogas generally impossible without acompressor. Further the area needed for the implementation of theapparatus, certainly when a primary settler is needed, is rather largeand not always available close by the waste-water source.

For low strength waste waters a modified UASB reactor has been designed,the so-called EGSB (Expanded Granular Sludge Bed) reactor (see G.Lettinga and L. H. Pol, Wat. Sci. Tech. vol. 18, no. 12 (1986) pp.99-108).

When low strength waste water is fed to a UASB reactor the gasproduction will be too low, the mixing due to this gas formed insidethis reactor is insufficient, and the reactor does not functionproperly.

In order to overcome this problem a significantly higher upward liquidvelocity is applied in the EGSB system, which results in a distinctsludge bed expansion and consequently in a better sludge water contactand therefore a better biomass use. Commonly these higher upwardvelocities are obtained by recirculation of the effluent.

However, the EGSB-concept is only suitable for treating relatively coldand very low strength waste water. When high strength waste water is fedto an EGSB-system the large amounts of gas formed in the reactor willdisturb the purification process. Part of the sludge particles will thenbe washed out of the reactor together with the suspended solids of theinfluent. In a conventional settler design these removed sludgeparticles are difficult to separate from the suspended solids, resultingin a loss of active biomass. Therefore an EGSB reactor will not functionunder conditions of a high strength waste water supply and thereforenormal UASB reactors are preferred under these conditions. In thisarticle of G. Lettinga and L. H. Pol no sludge growth is mentioned.

It is an object of the invention to provide a process in which theadvantages of the fluidized-bed process and the UASB-process into animproved fluidized-bed process are combined and results in an improvedfluidized-bed process.

It is a further object of the invention to provide an improvedfluidized-bed process which may be carried out in an improved apparatuswhich is very suitable for such kinds of process. In this apparatus notonly the process of the present invention may be performed, but otherfluidized-bed processes (using a carrier material) may be advantageouslycarried out as well.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows the three phase separator as disclosed inEuropean patent application EP-A90450.

FIGS. 2-4 schematically represent embodiments of the invention and thehorizontal cross-sections thereof illustrating the collection of part ofthe biogas, formed in the reactor.

FIG. 5 schematically shows the use of two parallel plates in thethree-phase separator.

FIG. 6 schematically shows the use of several parallel plates in thethree-phase separator.

FIG. 7 shows two embodiments of the construction of the liquiddistribution device and a horizontal cross-section thereof.

FIG. 8 shows a laboratory fluidized-bed reactor in which sludge granulescan be tested.

FIG. 9 schematically shows the three-phase separator which is providedwith several parallel partitions. This separator is tested onpilot-plant scale.

UASB-reactors commonly have a liquid residence time of 4-20 hours.Granular sludge in UASB-reactors is exposed to superficial liquidvelocities of 1-2 m/h and to superficial gas velocities (at the upperpart of the reactor) of 1-2 m/h. Surprisingly it has been found thatthis type of granular bed process with liquid residence times of 0,5-4h, superficial liquid velocities of even 4-25 m/h, preferably 620 m/hand superficial gas velocities of even 4-15 m/h without being damaged.

After extensive research and experimentation it has surprisingly beenfound that granular sludge not only keeps its granular structure andbiological activity after being properly introduced into thefluidized-bed reactor, the granular sludge accommodates to the much moreturbulent fluidized-bed conditions as well. Even the amount of granularsludge in the reactor increases in time. This phenomenon is found inacidification as well as in methane formation reactors. This improvedfluidized bed process has the benefits of the fluidized bed process aswell as of the UASB process, but does not possess the disadvantages ofthe processes in question. The results of the improved process fromlaboratory scale, pilot plant scale and full scale installations clearlyshow the benefits of this invention, not only during the starting-upperiod, but also under steady-state conditions.

The benefits of the process of the present invention over the EGSBprocess can be found by comparing the present process with the EGSBprocess as described by A. W. A. de Man et al. (Proc. 5th Int. Symp. onAnaerobic Digestion, Bologna, Italy, 22-26 May, 1988, p. 197 ff.), whichonly became public after the priority date of the present invention.

As described hereinbefore the EGSB is not suitable for high strengthwaste waters. The high superficial velocity in the reactor is obtainedby a high recirculation of the effluent. The capacity of the reactor inconsequence thereof is low. Low strength waste waters result in low gasproduction in the reactor, and therefore relatively gentle conditions inrelation to turbulence are present.

The insight that the loss of active sludge particles has to be preventedin processes with high gas and liquid velocities, not by choosing gentleconditions inside the (EGSB) reactor, but by choosing an efficient andselective return of the sludge particles to the reactor, forms the basisof the present invention.

Such a return of the sludge particles can be obtained using differentseparation techniques, e.g. settlers placed on top of the reactor orsettlers situated elsewhere. By choosing the proper conditions in such aseparator, sludge particles are separated from the effluent containingsuspended solids. In this way it becomes possible to purify waste waterswhich are totally unsuitable for EGSB processes.

In Table 1 some of the typical characteristics of the EGSB and thepresent process (UFB) are compared in order to show the benefits of thepresent invention.

                  TABLE 1    ______________________________________                              UFB                              (present                        EGSB  process)    ______________________________________    Height of the reactor (m)                           5-10   10-25    Gas production due to converted COD                          >1      1.5-15    (m.sup.3 gas/m.sup.2 reactor cross section.d)    COD load (kg COD/m.sup.3 reactor.d)                          1-5     10-80    Recirculation ratio*   5-10   0-4    Superficial liquid velocity (m/h)                          6-8      5-20    ______________________________________     *recirculation ratio = ratio between recirculated effluent of the reactor     and the influent of the reactor.

The starting up of the process of the invention can be accelerated byintroducing sludge from, for example, a UASB reactor or sludge fromanother reactor wherein the process according to the invention iscarried out. Starting up without inoculation of a substantial amount ofsludge is possible, however it will take considerable time before asufficient amount of granular sludge is present. An already operationalfluidized bed process using carrier material may be converted into thepresent process by choosing the right operational conditions. In thisway it becomes possible to switch gradually from the conventionalprocess to the present process. In an embodiment of the inventiongranular sludge as well as sludge on carrier is present. This embodimentcan be advantageously applied in already existing apparatus.

The granules of the present fluidized process are able to withstand thehigh turbulent conditions in the reactors. They show also otherexcellent qualities, the granules remain active for years and may beused for the starting up of other fluidized processes. Moreover they maybe advantageously applied in the purification of waste waters ofseasonal plants.

The terminal falling velocity of the sludge granules (which is a measurefor the settlement properties) is higher or at least equal to the sludgeobtained from UASB reactors. The activity of the sludge is at least ashigh as or often higher than the sludge of a UASB reactor. It has beensurprisingly found that the qualities of the sludge are dependent on theconstruction of the reactor and the three-phase separator, which will beexplained in more detail hereinbelow.

The improved fluidized bed process may be carried out in an unmodifiedfluidized-bed reactor, designed for purification processes using biomassattached to carrier, but the process of the invention is advantageouslycarried out in an improved fluidized-bed reactor, which is describedhereinafter in detail. Experiments on full scale have shown that withthe unmodified fluidized-bed reactors sub-optimal results are obtainedwhich are related to the construction of the liquid distribution deviceand the three-phase separator on top of the reactor.

The energy dissipation as a result of the liquid leaving thedistribution device (5-10 m/s) is high and to such an extent thatgranular sludge may be disintegrated by the force of the liquid spouts.The three-phase separator described in European patent applicationEP-A-90450 functions well during relatively low liquid and gas loadconditions, but at high loads (a fraction of) gas will often enter thesettling compartment. This will disturb the settling process and resultin a loss of overgrown or bare carrier particles, or natural granules.

Furthermore, the present invention provides improvements in relation tothe construction of the liquid distribution device, to make thefluidized-bed process more suitable for the use of granular sludge.

Moreover the construction of the 3-phase separator is improved,resulting in a reduced loss of overgrown or bare carrier particles, andin the case of the preferred embodiment of the invention even in a muchlesser loss of granular sludge. By doing so a higher concentration ofactive biomass in the reactor can be maintained, even at high gas andliquid velocities. This improvement will be demonstrated on the basis ofa known circular fluidized-bed reactor (1) which is shown in FIG. 1.Assuming a homogeneous distribution of the rising gas bubbles over thereactor cross-section, about 60% of the gas bubbles, formed in thecompartment below the throat (2), will be concentrated to form acylindrical bubble curtain (3). As a result of the sideward movementcaused by the deflection of the upward flow of liquid into the settler(4) at the throat, this bubble curtain is not stable. The radial overlapbetween gas collecting hood (5) and throat (2) is insufficient to leadaway all gas bubbles via the hood at any moment. Bubbles entering thesettler (4) then disturb the sedimentation process. In reactors withonly a relatively low gasflow in the upper part of the reactor thisproblem plays an insignificant role.

In U.S. Pat. No. 4,609,460 an apparatus is disclosed which collect aconsiderable portion of the gas developed by fermentation in an UASBreactor, before this reaches the upper part of the react(or. Theconstruction is rather complicated and exists of a lot of collectingsystems just above each other throughout the reactor, in connection withone or more downpipes. In such a system the liquid flows are difficultor almost impossible to control, even for UASB-processes. Influidized-bed systems they cannot be used at all.

In U.S. Pat. No. 4,622,147 an apparatus for a UASB-process is disclosedwith consists of three levels of gas-collection hoods. Just as in U.S.Pat. No. 4,609,460 this design is directed to a UASB reactor with lowliquid and gas velocities. Not only does this construction not solve theabove mentioned problem concerning the bubble curtain which will occurwhen applied in a fluidized-bed reactor, but this phase separationconstruction occupies in comparison with the reactor volume aconsiderable part of the reactor.

Another solution for this problem which would seem to be obvious, isenlarging the overlap between the throat and gas collection hood.However it was found that the liquid velocity increased in the throat ofthe reactor which made the return of settled biomassparticles out of thesettler more difficult. Also the larger diameter will lead to a largerhood, which is more difficult to construct.

Advantageously it was found that the bubble curtain which isconcentrated in the throat of the reactor can be separated from theflowing liquid. When the collected gasbubbles are sent directly to thegascollection space the problem of the bad functioning of the settler issolved, even for extreme high gas and/or liquid velocities in thereactor. This special biogas collection device may be mounted not onlyin new fluidized-bed reactors, but also existing fluidized-bed reactorsmay be supplemented with the apparatus of the invention.

Several embodiments of this separate biogas outlet are shown in FIGS. 2,3, 4 and 5. Partition (6) collects the gasbubbles which without thispartition would form the bubble curtain. Via tubes (7) or compartments(7) the collected gas is led directly to the gas collection space (9) orto a collection tube (8) (see FIG. 4) which is connected with the gascollection space. In a commercial full-scale circular reactor thediameter of the partition (6) may be around 0.5-10 meter and for examplein case of a diameter of the reactor of 3 meter, the height of thispartition may suitably be 30-90 cm. It will be appreciated that all thetubes have to be dimensioned in such a way that they will conduct thegases into the gas collection space and amply above the liquid level inorder to prevent the occurrence of an airlift circulation flow in thesettler.

The partitions (6) will collect 30-80% of the gas leaving the reactor,more preferably 50-80% of this gas. Although circular reactors arepreferred, square, rectangular or other shaped reactors are included inthe scope of the present invention. The fluidized bed reactor willconveniently have a height of at least 6 m, preferably at least 10 m.The ratio H/D will conveniently be 2-40, preferably 2-10 (H=height ofthe reactor, D=diameter or average cross-sectional dimension of thereactor).

Furthermore the present invention provides the use of parallel plates inthe three-phase separator, which stimulates a better coagulation andgrowth of small solid particles to larger granules. In FIG. 5 and 6 twoembodiments of such constructions are given. In U.S. Pat. No. 4,253,956a UASB apparatus is shown with interrupted inclined walls of the settlerto form such inlet openings. The wall parts thus formed are staggered atthe inlet opening to screen them from rising gas so that this gas cannotenter the settling compartment. An outlet opening for the settler at thelower end of the inclined walls makes a return of granules possible.Because of :the high liquid and gas velocities it is surprising thatthis principle can be used in a fluidized-bed. By bypassing asubstantial part of the gas formed in the reaction space it becomespossible to apply such a principle in a fluidized-bed process (incombination with the gas-collection device).

FIG. 5 shows such a partition (10) mounted in the settler. Around thispartition a circulation of liquid takes place, which results in a betterreturn of the settled particles from the settler into the reactor and animproved gas/liquid separation around the upper part of this partition(10). This partition may have a conical shape when a circular threephase separator is used.

Moreover rising biogas that will core under this partition, will becollected and will flow upwards, thereby creating an gaslift circulationaround this partition because of the difference of the density on bothsides of this partition.

In FIG. 6 partitions 10A and 10B are mounted as well which give rise toan even better functioning of the three-phase separator. The number ofpartitions depends on the volumes of liquid to be treated and technicaldesign dimensions. However it may be economically more attractive toincorporate just one partition (10).

It has been surprisingly found that gaslift circulation contributes tothe good and reliable operation of the reactor. In consequence of thiscirculation all liquid that will leave the settler, will circulate about5-20 times and pass through upward and downward movements. The meanresidence time of the particles in this zone increases and theopportunity of collisions will increase as well.

The small particles (granules) may be grown together by collisions orcoagulation and become larger particles. In this way the loss of activebiomass is reduced and the speed of the formation of granular,well-settable sludge increases substantially. Therefore it is possibleto separate the small active particles, which grow together, from thesuspended solids, which are not active in the purification process.

The conversion of smaller sludge particles into granular sludge isimproved as a result of the design of the three-phase separator.

It will be appreciated that this makes a fast start-up of the reactor,in comparison with the fluidized-bed system, possible even if only amoderate quantity of inoculation sludge is present.

In case that the cross-section of the 3-phase separator is chosen to becircular, the partitions (5) and (10) are preferably truncated cozies.

The liquid distribution device as described in EP-A-90450 is designedfor equal distribution of liquid into a bed of bare carrier-particles(e.g. sand). Due to the rather heavy mass of solids, and the need toprevent sticking together (then forming stagnant zones), the liquid flowrate through the spouts must be high (5-10 m/s) and to such an extentthat granular sludge may be desintegrated.

The present invention provides an improvement in relation with theconstruction of the liquid distribution device, to make thefluidized-bed reactor more suitable for the use of granular sludge (seeFIG. 7).

Natural granules have nearly no tendency to stick together when theliquid flow (when being stagnant) is interrupted. Surprisingly even arather thick layer (several meters) of granular sludge permits a simpleand easy start up of the fluidisation process by supplying the liquid ata few points in the lower part of the bed. Mixing, fluidisation andhomogenisation of the bed takes place at superficial (upflow) velocitiesof 6-10 m/h. The active, granular sludge starts gas production directlyin the lowest parts of the reactor, which stimulates also the mixingprocess.

One aspect of the present invention of which some examples are given inFIG. 7 consists of liquid-entrance pipes which are mounted in thereactor at several possible directions. There is no need for specialoutlet-nozzles that create a high local velocity. Normal outlet-velocityis 0.5-4 m/s, preferably 1-2 m/s. The liquid waste may therefore be fedto the fermentation zone through a plurality of mutually spaced inletsand optionally periodically interrupting or varying the flow of liquidwaste through each inlet. For example the liquid waste may be suppliedsuccessively through each inlet for a certain period of time. Bothrectangular and circular reactors may be used with flat or conicalbottoms.

EXAMPLE 1

On top of a laboratory scale fluidized-bed reactor (1) of 4 l and adiameter of 5.0 cm, a three-phase separator with a capacity of 2 l wassituated (see FIG. 8).

The fluidized-bed was inoculated with 2 l of biomass granulesoriginating from a UASB reactor, containing 150 g SS (suspended solids)or 120 g VSS (volatile suspended solids).

The superficial liquid velocity in the column was maintained during theexperiment at 8.8 m/h.

The liquid leaving the reactor via pipe (11) was partly recirculated(17.0 l/h) via pipe (13), the remaining part was discharged via pipe(14). Raw waste water (15) was introduced (2.1 l/h) together with therecirculated part of the effluent.

The gas formed was collected in chamber (9) and discharged via pipe(12).

A typical waste water containing 2000 mg/l of acetic acid and 480 mg/lof ethanol was purified in the fluidized bed reactor. Nutrients wereadded to stimulate the biomass growth.

During a period of 3 months including the starting up period, theaverage load was 36.6 kg COD/m³ reaction volume per day and theconversion-efficiency gave CODt=92.4%. Average production is 45.7 l/d ofbiogas (CODt=total Chemical Oxygen Demand). The superficial gas velocityis calculated to be 0.97 m/h in the upper part of the reactor.

The mean hydraulic residence time in the reactor was 2 h, the contacttime in the active fluidized-bed section was 6 minutes. At the end ofthe experiment 156 g SS, corresponding to 135 g VSS was present,corresponding to an expanded volume of 2.11 l.

The experiment proved that the sludge granules remain intact, and evenshow a nett increase of biomass (15 g VSS) under these conditions. Theprocess operates very stably with high efficiency.

EXAMPLE 2

Industrial waste water originating from the chemical and fermentationplants of Gist-brocades, Delft, was purified in a pilot scale process.the waste water was acidified in a continuous flow stirred tank reactor:the hydraulic retention time is 8-12 hours. the effluent of this reactoris fed to a pilot-scale fluidized bed reactor. The influent of thefluidized bed reactor contains 1800-4500 mg/l COD_(t), 350-500 ng/lsulphates and 0.5-1.0 g/l inert-SS. On an average this influent isacidified for 50%-80%, calculated for fatty acid contribution in thedissolved COD. A cylindrical fluidized bed reactor was used as shown inFIG. 1. The height of the reactor (without the three-phase separator)was 19.45 m, the diameter 0.495 m resulting in a useful volume of 3.7m³. the smallest cross-section area near the throat (2) was 0.031 m²,the corresponding cross-sectional area of the gas collecting hood at thelowest end was 0.108 m², the angle of the cover hood part was 55°relative to the vertical.

150 kg DS (dry solids) of granular anaerobic sludge was added to thereactor. The sludge originated from an anaerobic UASB-reactor which wasused to purify waste water of a sugar mill. The content of organicmaterial of the sludge was 82%, corresponding at the reactor start upwith 120 kg VSS. The sludge granules had an average dimension of 2-3 mm.At the start of the test the height of the expanded granular sludge bedin the reactor was 7.7 m.

The acidified waste water together with the recirculated part of theeffluent was introduced at the bottom of the reactor. The pH of theacidified waste water was 6.0-6.7 and the pH of the recirculated liquidwas 7.2-7.5.

1.40 m³ /h liquid was introduced into the reactor, corresponding with asuperficial liquid velocity of 7.4 m/h. The percentage of acidifiedwaste water of the introduced liquid was increased stepwise; 300 l/h onday 1 till 1000 l/h on day 5 acidified waste water was fed to thereactor. On day 5 the total amount of liquid introduced was increased to1.80 m³ /,h, corresponding with a superficial liquid velocity of 9.5m/h. The step wise increasing of the load was carried out on the basisof the fatty acid content in the effluent, which was kept less than 100mg/l.

In Table 2 the quantities of acidified waste water and total quantitieswaste water are given, and the superficial liquid velocities.

                  TABLE 2    ______________________________________         acidified           total   superficial                                            hydraulic         waste water                   recirculated                             liquid  liquid residence         introduced                   waste water                             introduced                                     velocity                                            time    day  (m.sup.3 /h)                   (m.sup.3 /h)                             (m.sup.3 /h)                                     (m/h)  (h)    ______________________________________     1   0.3       1.1       1.4      7.4   12.3     5   1.0       0.8       1.8      9.5   3.7     3   1.3       0.5       1.8      9.5   2.9    26   1.5        0.65      2.15   11.4   2.5    34   1.7        0.75      2.45   12.9   2.2    39   2.1        0.35      2.45   12.9    1.75    43   2.5       0.4       2.9     15.3    1.48    ______________________________________

During the test which lasted 50 days the temperature of the reactor was30°-34° C.

The efficacy of the reactor remained during the period stabile, theefficiency of the removal of fatty acids was all the time ≧, 90%.

During the test the height of the expanded granular sludge bed wasfrequently measured. At the end of the test the height of the granularsludge bed was 4.6 m. The granules were well-settable and had an averagedimension of 2-3 mm. The biogas production flow rate average was 40 m³/day, corresponding with 20 kg COD/m³.day conversion and a superficialgas velocity of 8.5 m/h in the top of the reactor. At peakloads thegasflow rate even was about 14.5 m/h.

Because inoculation sludge originating from an anaerobic UASB-reactorwas added, the sludge in the beginning of the experiment consisted inpart of small particles which were at least partly washed away. Thisresulted in a loss of granular sludge at the beginning of theexperiment. After this period the amount of granular sludge stabilized.

After the test about 84 kg (total DS) of granular sludge was presentwith an organic material content of 85% corresponding with about 71 kgVSS. The test demonstrates that most granules remain intact, even at thehigh biogas and liquid flow rates and, accomodate rapidly to thevariable influent-qualities. The granules became a more compact shapeduring the test; the specific activity of the biomass has a mean valueof about 0.8 kg COD/kg. VSS per day (at peak loads=1.6)

EXAMPLE 3

On top of the circular fluidized bed reactor of Example 2 (height 19.45m and a diameter of 0.495 m) a three-phase separator according to FIG. 9was mounted. This three-phase separator consists of a rectangularcompartment (16) of which one side has a semi-circular form (17). Theunderside of this compartment is provided with an adaptor (18) whichjoins a cylinder (21) having a diameter of 0.495 m, which is connectedwith the reactor. Partition (6) will collect about half of the biogasformed in the reactor, performing the same function as described for thedevices (6) in FIGS. 2, 3, 4 and 5. This collected biogas is led throughtube (7) directly to the gas collection compartment (9). Beneath the gascollection hood (5), four partitions (10) are mounted.

The total volume of the three-phase separator was about 0.75 m³. Afterhaving passed the airlift-circulation-flow the liquid will enter thesettling compartment (4). The effluent leaves the reactor, after passingweir (19) (overflow), through outlets (20). The collected gas is removedvia outlet (21). In FIG. 9 two liquid outlets (20) are shown.

Waste water originating from Gist-brocades, Delft (see Example 2) waspurified in the fluidized bed reactor. Identically to Example 2 thiswaste water was first acidified. In the beginning of this test thereactor was filled with 84 kg granular anaerobic sludge (=71 kg VSS),which was present at the end of the experiment described in Example 2.In the period of 12 days between the tests of Examples 2 and 3, thesludge was kept in the reactor. This stop did not influence thebehaviour or activity of the sludge.

On day 1 500 l/h acidified waste water together with 2.2 m³ /hrecirculated liquid was introduced into the reactor, corresponding witha superficial liquid velocity of 14 m/h. On the next days the quantityof acidified waste water was increased, meanwhile keeping the totalquantity introduced liquid into the reactor constant. Therefore on day2: 1000 l/h; on day 3: 1500 l/h; on day 4: 2000 l/h; and from day 3:2500 l/h of acidified waste water was fed. Daring this period of timethe biogas production increased from about 8 m³ /day till maximal 75 m³/day (peak load). The fatty acids-COD content in the purified wastewater was always below 150 mg/l. This clearly demonstrates that thepurification was substantially complete. The gross load, calculated onbasis of the useful volume of the reactor, varied between 30 and 95 kgCOD/m³.day corresponding with a conversion of 12-40 kg COD/m³.day. After70 days of continuous operation these experiments were stopped; afterdegassing the amount of granular anaerobic sludge was 103 kg and theamount of granular biomass was estimated to be 88 kg VSS.

The tested 3-phase separator demonstrates a substantially positiveinfluence on the growth of anaerobic (methanogenic) biomass intogranules, even at high liquid and biogas-velocities (liquid V_(sup) ≅14m/h, biogas V_(max) in top of reactor≅16 m/h).

EXAMPLE 4

The circular fluidized bed reactor as used in Example 3 (with threephase separator according to FIG. 9) was used to study sludge growth andgranulation.

Waste water originating from Gist-brocades, Delft (see Example 2) waspurified in the fluidized bed reactor. Identically to Example 2 thiswaste water was first acidified (Hydraulic Retention Time=3-4 hours).

Contrary to the other experiments the pilot plant would be started witha rather small amount of granular sludge. An increase in sludge bedheight, combined with measurements on amount of biomass (TS, VSS) andamount of sludge particles should prove net sludge growth and formationof new sludge particles (=granulation). Moreover sludge activity testsshould prove the presence of active biomass. The reactor was startedwith 1.5 m sludge bed height (14 kg VSS, 70% VSS, 20 kg TS). Therecirculation flow (raw influent + recycled effluent) was 14.4 m³ /dayduring the total experiment, resulting in a superficial upflow velocityof 5 m/h. The raw waste water flow was slightly increased from 200 l/hat the start to 600 l/h after 2 weeks. This did result in a COD loadingrate of 60-80 kg COD/day (=16-22 kg COD m³ /day). The biogas productionvaried between 8-14 m³ /day resulting in a superficial gas velocity ofapproximately 2-3m/h. The COD purification efficiency was 55-60%.

After three months the sludge bed height had increased from 1.5 m to 4.3m. The total amount of sludge increased from 20 kg TS to 49 kg TS, whilethe amount of organic sludge increased from 14 kg VSS to 40 kg VSS. Thesludge activity at the start was 0.9 kg COD/kg VSS. day and 1.2 kgCOD/kg VSS.day at the end of the 3 months experiment. Measurements onindividual sludge particles showed a 190% increase in the amount ofgranules.

The results of this experiment show clearly that with the settler deviceaccording to the invention granulation and net sludge growth isobtained. Besides it is shown that a sludge with a high content ofactive biomass is found indicating that non valuable suspended solidsare not retained in the reactor.

EXAMPLE 21

This example describes a full-scale 2-step anaerobic treatment of wastewater originating from a yeast producing factory. This waste water was amixture of waste water, distillate of an evaporator and filtrate of theyeast filters.

The average influent contains: 2500-4000 mg/l COD, 300-600 mg/l SO₄ ²⁻and 300-600 mg/l suspended solids.

The three fractions were buffered and mixed in two tanks connected inseries, each having a volume of 100 m³. The temperature in the buffertanks was about 37° C. At an average hydraulic residence time of 5-7 hthe waste water was partly acidified, about 60-90% of the biodegradableCOD was acidified. The two reactors, connected in series, are identicalto each other; their three phase separator construction is shown in FIG.1.

The reactor is described as well in European Patent Application 0090450(see FIGS. 2, 7 and 8). The height of the reactor is 12.3 m (withoutthree-phase separator), the diameter of the reactor is 3.0 m, thediameter of the three-phase separator is 4.0 m. The useful volume of thereactor is 80 m³. The ratio between the parts of the three-phaseseparator is substantially the same as the ratios of the correspondingparts of Example 1.

The waste water to be purified was introduced via 5 horizontaldistribution pipes, provided with downwardly directed liquid entrancenozzles.

At the beginning of the test in the first reactor (R-1) 5000 kg sand andin the second reactor (R-2) 13500 kg sand was present as carriermaterial. This carrier material had an average diameter of 0.2-0.4 mmand a bulk density of 2700 kg/m³. The sand was overgrown with biomasswhich was in principle capable of purifying the waste water. In thefirst reactor mainly the acidification and in the second reactor mainlythe methane-fermentation took place. However due to operationalproblems, as described before, in the months preceding the test asubstantial amount of overgrown sand was washed away, which caused atthe same time loss of a considerable quantity of biomass. The operationof the system was therefore unstable, keeping in mind that the initialload of both reactors was 40000 kg sand.

At day 1 the operational conditions were changed in such a way that theformation of the granular sludge according to the invention wasstimulated whereby sand was not used as carrier anymore. This has beenachieved by taking away the remaining sand stepwise and decreasing theamount of total liquid introduced of 100 m³ /h to about 65-70 m³ /h,corresponding with the decrease of the superficial liquid velocity of 14m/h to 9 m/h. The amount of acidified waste water introduced at thebeginning of the experiment was 20 m³ /h. This quantity was increased to40 m³ /h at day 76. The total amount of liquid introduced into thereactor was maintained constant by decreasing the quantity ofrecirculated liquid. At day 1 the fatty acid conversion was incompletealthough a moderate load was applied: the effluent of the R-1 (firstreactor) contained 1900 mg/l and R-2 (second reactor) 600 mg/l (fattyacid efficiency is 67% in R-2). During the test which lasted 7 months,the quantity of granular sludge gradually increased in both reactors.During this whole period the conversion efficiency increased to themaximum fatty acids conversion, and the influent load could be increasedtill all the waste water was treated. The process showed a stablebehaviour and the problems met during the operation with sand ascarrier, disappeared by applying the process of the invention.

Table 3 shows the results during the testrun.

                  TABLE 3    ______________________________________           Reactor 1                    Reactor 2   load R-2 efficiency              biomass       biomass (kg COD/m.sup.3) R-2         influent       granules    granules        fatty         flow    sand   (kg   sand  (kg        COD  acids    day  (m.sup.3)                 (kg)   VSS)  (kg)  VSS)       (%)  (%)    ______________________________________     1   24      5000    130  13500  70   16.5 50   67     41  30      4000    190  13000  160  14.5 74   78     76  40              360         830  18.5 62   80    127  40      1000   1370   8000  730  17   62   79    132  41       200   1500   4000 1500  20   65   82    200  41       200   2000   4000 2000  20   65   32    ______________________________________     *kg VSS = kg volatile suspended solids.     The biomass quantity is calculated on basis of 4 samples taken at     different heights from the cylindrical reactor.

The activity of the granular sludge, measured under substrate saturatedconditions is shown in Table 4.

                  TABLE 4    ______________________________________         activity of the                      activity of the                                 activity of         granular sludge                      granular sludge                                 granular sludge         in R-1       in R-2     washed out with         (kg COD/     (kg COD/   the effluent    day  kg VSS · day)                      kg VSS · day                                 (kg COD/kg VSS · day)    ______________________________________     73  0.8          1.98       2.25    185  0.85         1.63       1.42    ______________________________________

The activity of the granules originating from reactor 1 is lower thanthose from reactor 2 because in reactor 1 acidification as well asmethane-forming bacteria are present. The activity of the granularsludge of reactor 2 is high compared to usual values in anaerobicsystems (such as USAB-reactors) of 0.4-0.3 kg COD/kg VSS.day. Due to theimperfect three-phase separator part of the sludge is washed out withthe effluent. The granules washed out, appeared to be very active aswell. The terminal falling (settling) velocity of sludge granules takenat day 155 from reactor 1 at a sample point 1 m above the liquiddistribution system was 39 m/h, at a sample point 3.9 m above the liquiddistribution system was 30 m/h and at a sample point 7.3 m above theliquid distribution system was 27 m/h.

For reactor 2 the terminal falling velocity of the granules was 32 m/hat a height of 1.0 m and 29 m/h at a height of 3.9 m above the liquiddistribution system.

As demonstrated above it is possible to obtain granular sludge withoutan inoculation of similar sludge material in fluidized bed reactorswherein the carrier material (such as sand) has virtually disappeared.

The upward superficial liquid velocity was 8-10 m/h, the superficialbiogas velocity in the upper part of the reactors was 2-5 m/h (R-1) and3-8 m/h (R-2), respectively. Although the reactors were not suppliedwith the above described improvements viz, the gas collection partitions(6) or partition (10) or the simplified influent pipes, granular sludgewas formed, which was able to purify the waste water.

The result is notable in view of the industrial circumstances of thefactory viz. the continually changing liquid and COD-load, for examplein the weekend hardly any waste water was supplied to the system.

EXAMPLE 6

The special gas collection device, as described before on the basis ofFIG. 4 was installed in two fullscale reactors with circular diameter of5 m and diameter of hood of 6.5 m; grossvolume is 380 m³, and nettvolume of the reactors is 240 m³. The reactors operated in series, i.e.the feed liquid was introduced in Reactor 1 in which mainlyacidification and sulphate reduction processes take place, thereafterthe "acidified" liquid was introduced into Reactor 2 in which mainlymethanisation processes take place. The special gas collection devicecollected about 70% of the gas formed, which gas did not enter thethree-phase separator.

Parallel with the two mentioned reactors, two other reactors operatedwith exactly the same influent composition, same biological processesand identical technical dimensions of the reactors and identicaloperational conditions. In the second set of reactors however thesespecial gas collection devices were not installed. All (4) reactorsoperated on the biomass-on-carrier principle, using sand (0.2-0.4 mm) ascarrier. A period of over 500 days in which the two sets were operatedwithout main interruptions demonstrated clearly the positive effect ofthe gas collection device. Without this device the loss of overgrownparticles with a terminal falling velocity of 60 m/h starts already atliquid superficial velocity of 10 m/h and amounts then up to about 50kg/day. At a superficial velocity of 16 m/h the loss increases to 200kg/day and more. With the special gas collection device the loss ofidentical particles is virtually nil even at a superficial velocity of16 m/h.

We claim:
 1. A three-phase separator for a fluidized bed apparatus forthe anaerobic treatment of a waste water liquid phase wherein a gasphase is generated, the three-phase separator being adapted to besituated in the top of a reactor for separating the gas phase from atreated liquid phase, and from a biomass, and for returning the biomassto the reactor, the three-phase separator comprising:a settlingcompartment having an inlet, and outlet for treated liquid phase fromwhich the gas phase and biomass have been substantially removed, aplurality of parallel partitions connected to the settling compartmentand inclined to both the vertical and the horizontal for creating aninternal gaslift circulation of biomass and treated liquid phase in theseparator, the parallel partitions having an inlet for the biomass andtreated liquid phase that is separated from the gas phase and flowsbetween the partitions to an outlet from said parallel partitions, theparallel partitions inlet being above the parallel partitions outlet,and the outlet from the parallel partitions being in open communicationwith the inlet to the settling compartment for the flow of treatedliquid phase into the settling compartment, a deflector inclined to thevertical and the horizontal, said deflector being connected to thesettling compartment and having a major surface extending below andacross the inlet to the settling compartment and below and across theoutlet from the parallel partitions to provide an outlet from theseparator for return of the biomass from the separator to the reactor,the outlet for the biomass being spaced apart from the inlet to theparallel partitions, the inclined deflector and the parallel partitionsbeing at opposite inclined angles with respect to each other, wherebytreated liquid phase flowing into the settling compartment and biomassis separated from the treated liquid phase and flows back to the reactorwith the aid of the gaslift circulation.
 2. The three-phase separator ofclaim 1 wherein the parallel partitions are oriented at an angle to theinclined deflector.